Canine transmissible venereal sarcoma (CTVS) has been thought for many years to have a viral etiology, but to date no evidence to support this belief has been demonstrated. The present work will attempt to study by recently developed methods: (1) the activation of a sarcoma genome; and (2) the role of antigenic heterogenization of the tumor by viruses in spontaneous regression after a period (2-4 months) of rapid growth. Isolation and identification of an agent which causes dog sarcoma promises to contribute greatly to an understanding of the viral etiology of sarcoma in general. As in feline leukemia, the oncogenicity of the rescued agent could be studied in an original host species which lives in the same environment as man.